Contributions to Zoology, 70 (3) 175-179 (2001) SPB Academic Publishing bv, The Hague Short notes and reviews classification: of the Simplifying hydrozoan inappropriateness group Hydroidomedusae in a phylogenetic context Antonio+C. Marques Departamento de Biologia, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo. Av. Bandeirantes, 3900, 14040-901, Ribeirão Preto, SP, Brazil, e-mail: [email protected] Keywords:: Hydrozoa, Hydroidomedusae, classification, nomenclature, taxonomy Abstract ated classifications and models of phylogeny si- multaneously. In phonetics, classifications reflect The systematics ofHydrozoa is consideredfrom the viewpoint overall similarity, not necessarily phylogeny (Hull, of logical consistency between phylogeny and classification. 1988: 122; Mayr and Ashlock, 1991: 128). With The validity of the nominal taxon Hydroidomedusae (includ- regard to the higher levels of hydrozoan classifi- all of the is dis- ing groups Hydrozoa except Siphonophorae) cation, no cladistic or phenetic classifications have cussed with regard to its distinctness and inclusive relationships. In been proposed. All classifications seem general, phylogenetic systematic evidence suggest that the hydrozoan use of the is term Hydroidomedusae inappropriate given our to be based on gradistic or essentialistic principles, current level of It is concluded that understanding. no new, or although the majority of their proponents would names until broader resurrected, are necessary before or a not characterize them as such. phylogenetic revision ofthe Hydrozoa is accomplished. In classification should my view, a represent one of the possible hypotheses of phylogeny (cf. the principle of logical consistency by Hull, 1964). In Introduction other words, “if the cladogram that follows the true phylogeny (assuming that the true phylogeny could Classification of is organisms a way to “systema- be known) is the same as the cladogram expressed tize” biological information within some kind of by a classification, then the classification is clearly logical framework. Therefore systematics could be ” consistent with phylogeny (Platnick, 1979: 542; the viewed as science of classifying or organizing For my italics). Hydrozoa, Bouillon (1981, 1985) organisms into a hierarchical perspective. By the provides examples of a classification logically term information biological we mean any prop- consistent with proposed hypotheses ofphylogeny, erty intrinsic to the organism, such as ecology, although these phylogenies were constructed with- morphology, molecular constituents, behavior, etc. the out use of rigorous phylogenetic or cladistic Hence, taxa formed only by convenience (e.g., methods. Petersen (1990), Marques (1996), Pena economical assuming importance or by the per- Cantero & Marques (1999), and Marques & Migotto sonal preference of its proponent) ought to be dis- (2001) provide examples of phylogenetic and couraged. cladistic approaches as a basis for the classifica- Darwin (1859) effectively focused the attention tion of The aim of this is to dis- of hydrozoans. note biologists to reflect on issues of descent and cuss in light ofknown aspects of the phylogeny of divergence from common ancestors. Subsequently, the the for a taxon workers Hydrozoa justification “Hydro- like Julian Huxley, Ernst Mayr, and George idomedusae” by Bouillon et al. (1992). G. in Simpson, what came to be called the “Mod- ern Synthesis”, accommodated both grades and clades (cf. Mayr & Ashlock, 1991). Cladistics excluded and (Hennig, 1950, 1966) grades gener- Downloaded from Brill.com10/07/2021 03:16:02AM via free access 176 A. C. Marques - Hydroidomedusae phytogeny classification The Hydrozoan uniqueness of such groups implies that they represent monophyletic lineages that are distinct delineated of A historical review of hydrozoan classifications by a set autapomorphies. Are the was compiled and discussed by Bouillon et al. Siphonophorae monophyletic vis-a-vis (1992). From a methodological perspective, all the other hydrozoans? This is not clear in Bouil- proposed classifications for the Hydrozoa have been lon et al. (1992), and I can see no justification other than that essentialistic, i.e., focused on grades of evolution. ofhistorical convenience. Students of the Bouillon al. also observed that do not deal with et (1992) most pre- Hydrozoa generally siphonophores, derived vious classifications have not been adequate. Al- possibly because of their very morphol- and life histori- though these classifications based their arguments ogy, ecology, history. Obviously, of historical cal on the assembling taxa within con- convenience is not enough of a reason to separate cepts of some proposed names; the idea that a clas- a group. Perhaps the most striking difference in sification should represent some actual phylogenetic the siphonophore lineage is its “high degree of deter- 1987: knowledge has not been considered. As a substi- minancy of form” (Mackie et ah, 110-113). tution for those former inadequate classifications, The modular construction of siphonophores is based Bouillonet al. (1992) resurrected the name Hydro- on definite growth zones along an anterior-poste- idomedusae (originally proposed by Claus, 1877a, rior axis. Derivatives of both polypoid and medusoid 1877b) stating, “Our proposal is to divide the class stages are found attached to the axial free-floating Hydrozoa into two subclasses, the Hydroidome- animal (Kirkpatrick & Pugh, 1984). Observing the the 1992: dusac and Siphonophorae” (Bouillon et ah, singular morphology of the Siphonophorae, one 282). Thus, within the Class Hydrozoa, the siphono- might conclude that this group is highly modified and distant from all phores would be juxtaposed against all other hy- so, phonetically other Hydro- such drozoans. Although Bouillon et al. (1992) did not zoa. Indeed, phenctic classification reflects justify the existence of such a subclass Hydroido- similarity relationships, which are not necessarily medusae with defined several the result any clearly features, congruent with, or of, rigorous phylo- authors have followed al. them, e.g., Pages et (1992), genetic relationships. Pages & Gili (1992), and Boero et al. (1996). Oth- However, could siphonophores have a common with ers prefer to use the term Leptolida for this same putative ancestor some other hydrozoan sub- Cornelius Under such a grouping, e.g., (1995), Vervoort (1995), hypothesis, following an of the andBrinckmann-Voss & Arai (1998), a name whose event cladogenesis, siphonophore lineage underwent accelerated use was already denied by Bouillon et al. (1992). an process of anagenesis and/ Still others simply still consider Hydrozoa to sim- or specialization of form, resulting in a long and include several unnested Schuchert differentiated In this Petersen ply groups, e.g., highly lineage. regard, (1998) and Collins (2000). (1979: 109 more explicitly; 1990: 105) suggested Since Darwin hierarchical classifications indi- a possible relationship between Siphonophorae and cate relationships of subordination within nested the Anthomedusae, Leptomedusae, and Limnome- sets. Since Hennig, we focus on trying to identify dusae. Schuchert (1996: 13) was even more expli- natural that reflect cit in his that “affinities monophyly, groups common statements, considering ancestry. If we presume that the label “Hydrozoa” [of Anthomedusae] with the Siphonophora and (here considered a monophyletic group) is hypoth- Laingiomedusae are not established. All three taxa have the manubrium esized to be derived from a single ancestral line- gonads on as a possible synapo- then all descendants could be in Desmonemes in the age, grouped morphy. occur only Siphono- subordinate If of these and Anthoathecata and patterns. any subgroups are phora [= Anthomedusae] indicate A joined together under a taxon name, e.g., the Hydro- may a closer relationships. taxon Hydro- idomedusae of Bouillon et al. (1992) and posed idomedusae (Bouillon et ah, 1992) which includes against another, i.e., the Siphonophorae, this would all Hydrozoa except the Siphonophora is there- ” then necessarily imply that members of these sub- fore most probably not a natural taxon (my ital- groups are related and share a common lineage. ics). Downloaded from Brill.com10/07/2021 03:16:02AM via free access Contributions to Zoology, 70 (3) - 2001 177 I. the Anthomedusae and after Fig. Relationship among Leptomedusae, Siphonophorae, Marques (1997). in the manubrium reversal Indeed, Marques (1997), a phylogenetic study gonads on (a from gonads of the Cnidaria (Fig. I), concluded there is a relation- on the radial canals in the more inclusive subgroups, ship among Leptomedusae, Anthomedusae, and and another homoplasy with Laingiomedusae). Fig. 1 The Siphonophorae. Limnomedusae, with no auta- also shows the autapomorphies of Leptomedusae, pomorphy, are a group of uncertain monophyly. Anthomedusae and Siphonophorae. It is interest- form a clade with the other to note that the Anthomedusae They might possibly ing are weakly sup- hydrozoan subgroups (Trachymedusae, Laingio- ported as a subgroup, since one of the putative medusae, Actinulidae and Narcomcdusae). How- autapomorphies (perradial mesenteries in the that is A is ever, issue not relevant here. single group medusa [Petersen, 1990]) not accepted by some composed of the Leptomedusae, Anthomedusae, authors (e.g., it is not cited in Schuchert, 1996), and Siphonophorae is supported by the following and the other (arrangement of the polypoid